This invention relates to negative amplifiers of a pulse width modulated (PWM) type, and, in particular, to such amplifiers with stabilized feedback operation.
A known amplifier of such a type generally has a circuit formation as shown in FIG. 1. The input signal is voltage-compared at a voltage-comparator 2 with a triangular wave signal from a triangular wave generator 1 after being added a negative fed-back signal. The triangular wave has a higher frequency than the input signal to be amplified. Therefore, a PWM signal is provided from the voltage-comparator 2. The PWM signal is power amplified at a pulse amplifier 3, the output from which is demodulated at a low-pass-filter 4 to provide an amplified signal to a load 5. A part of the output voltage is fed back through a negative feedback circuit 6.
In this case, the negative fed-back signal is taken out from a point in the filter 4, not the output point of the amplifier 3, because the output of the amplifier includes a high voltage pulse wave component.
The low-pass-filter is one of a lumped constant circuit type comprising at least one stage LC-filter. The fed-back signal is taken out from a common connection point of an inductor and a capacitor of the LC-filter.
FIG. 2 shows a circuit of two stage LC-filter comprising inductors L.sub.1 and L.sub.2 and capacitors C.sub.1 and C.sub.2, the load 5 being also shown.
The PWM-type amplifier can be equivalently represented as shown in FIG. 3, wherein triangular wave generator, voltage comparator and pulse amplifier are represented by an amplifier A, because it is thought to be similar as a linear amplifier, except the formation of a PWM-signal. In FIG. 3, this difference is shown as an application of a rectangular pulse energy P at the output of the amplifier A.
Referring to FIG. 3, the low-pass-filter 4 is formed with one stage LC-filter comprising an inductor L and a capacitor C, from a common connection point of which the fed-back signal is taken out.
Now, the stability of the feedback loop will be stimulated, using Nyquist diagram thereof.
Assuming the input signal voltage V.sub.i, the resistance R.sub.L of the load 5, the feedback factor (or the ratio of voltage fed-back/output voltage) .beta. of the feedback circuit 6, the fed-back signal voltage V.sub.F, the voltage gain A of the amplifier A, and values L and C of the inductor L and the capacitor C, the transfer function of the open loop from the input terminal T.sub.i to the output of the feedback circuit 6 is given by following equation; EQU V.sub.F /V.sub.i = A.multidot..beta. 1 /CLR.sub.L S.sup.2 + LS+ 1 . . . (1)
, where S is a complex frequency used in Laplace transformation.
Since A.multidot..beta. can be considered as a real number in practical use, the vector locus of the equation (1) is drawn as shown in FIG. 4. Referring to FIG. 4, it is noted that the circuit in FIG. 3 is not stable, although does not oscillate, because the locus passes closely near the point (-1, jO). Furthermore, it will be noted that the transfer function of the closed loop has a peak, or is represented as a transfer function of the second order. Therefore, the negative feedback operation is not stable in the known PWM-type amplifier.